1. Field of the Invention
The invention generally relates to a driver circuit in a backlight system for powering fluorescent lamps, and more particularly, relates to a driver circuit that combines the advantages of a push-pull switching topology and a full-bridge switching topology.
2. Description of the Related Art
In liquid crystal display (LCD) applications, backlight is needed to illuminate a screen to make a visible display. A number of conventional inverter topologies (e.g., active clamping forward, phase-shifted full-bridge, resonant full-bridge, asymmetric half-bridge, push-pull, etc.) facilitate zero voltage or zero current switching to minimize switching stresses and losses. Among these conventional inverter topologies, the full-bridge topology and the push-pull topology are acceptable for cold cathode fluorescent lamp (CCFL) inverter applications because of their capability to produce symmetric lamp current waveforms.
Both the conventional full-bridge topology and the conventional push-pull topology have advantages and disadvantages for the CCFL inverter applications. The conventional full-bridge topology has an ability to control circuit behavior at all times. For example, a short circuit can be placed across a primary winding of a transformer in the conventional full-bridge topology when drive voltage is not applied to the primary winding. The conventional full-bridge topology advantageously preserves stored energy in the transformer or in inductor-capacitor (or tank) circuits.
In contrast, the conventional push-pull topology sometimes looses direct control of circuit behavior. For example, an open circuit is created within positive and negative power supply limits at primary windings of a transformer in the conventional push-pull topology when drive voltage is not applied to the primary windings. The conventional push-pull topology allows stored energy in the transformer and any tank circuits to leak back into primary winding circuits, often creating voltage spikes across switching transistors coupled to the primary windings. The cycle of energy storage and loss repeats for each cycle of the drive voltage. However, the conventional push-pull topology advantageously requires fewer driving control signals than the full-bridge topology, introduces less power loss in a power-delivering path and has fewer components.
The conventional full-bridge topology, on the other hand, generally has more complicated driving circuitry and is less power efficient. For example, the conventional full-bridge topology drives a set of upper switches and a set of lower switches. The upper switches and the lower switches often use different levels of gate drive control signals. In addition, the on-resistance of the upper switches appears as an I2R power loss in the power-delivering path.